Pre-stressed grids for electron tubes



NOV. 18, 1958 5, BROWN ET AL 2,861,211

PRE-STRESSED GRIDS FOR ELECTRON TUBES Filed May 31, 1955 Inventors K 5. BROWN- N. BASNETT By A Home y United States Patent Ofiice 2,861,211 PRE-STRESSED GRIDS ron ELECTRON TUBES Kenneth Stanley Brown, Castle Hill, New south Wales, and Thomas Nathaniel Basnett, Cheltenham, New South Wales, Australia, assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application May 31, 1955,.Se rial No. 512,238

Claims priority, application Australia June 29, 1954 9 Claims. (Cl. 313-s4s This invention relates to pre-stressed grids of planar type-for use in electron tubes, to methods of making such grids, and to electron tubes containing such grids.

A difficulty associated with the manufacture and operation of an electron tube having close spacing between a grid and some other electrode, when energy is dissipated on the grid, is that distortion due to heating of the grid may result in a contact between the grid and the other electrode, rendering the electron tube inoperative, and at times causing its destruction. This difficulty arises particularly in the case of thermionic vacuum tubes in which a planar grid and a cathode are closely spaced, but can arise in the case of other tubes such as gas-filled thermionic tubes and cold cathode tubes.

Various constructions have previouslybeen proposed for the grid of an electron tube to overcome this difficulty. In the case of a thermionic vacuum tube having a planar grid it has been proposed to stretch a number of parallel wires over a frame and to apply a tension to each wire to hold the wire straight. The maximum energy which can be dissipated on the grid depends on the tension in the wires since when the tension is reduced to zero the wire begins to distort, and it is just as likely to distort in the direction of the cathode as away from it. In the case of vacuum tubes having cylindrical grids in which the wires constituting the grid consist of straight wires disposed parallel to the axis of the cathode, it has been proposed to construct the grid with wires which are curved so that, while they lie on the surface of a cylinder, they are curved into part of a spiral about the surface; with this construction the wires will, as they distort due to expansion, move away from the cathode.

The object of this invention is to provide a planar type of grid having a plurality of straight wires stretched on a frame such that, if the grid wires overheat, the grid wires will move away from the electrode, which will usually be the cathode, from which they are closely spaced.

According to this invention a planar type of grid for use in an electron tube where the grid is closely spaced from "another electrode comprises a frame and substantially straight grid wires secured to the same, the grid wires being so stressed that, whilst they remain substantially straight while under tension, they will deflect in a desired direction away from the direction of the said closely spaced other electrode if the tension becomes reduced or the wires break.

Also according to this invention a method of making a grid for use in an electron tube where the grid is closely spaced from another electrode comprises the steps of securing substantially straight grid wires to a frame so that they be substantially in a common plane, and stressing the wires adjacent to the frame and tensioning the wires so that, while the wires between the frames remain substantially straight over the greater part of their length when under tension, they will deflect away from I drawings wherein:

Figure 1 is a plan viewof a planar grid and support;

Figure 2 is a sectional elevation of the grid of Figure 1 before forming;

Figure3 is a sectional elevation of the grid of Figure 1 after forming; and

Figure 4 is a sectional elevation of an alternative grid constructed in accordance with the invention.

In the embodiments of the invention illustrated by the drawings the grid is shown as circular in plan, the grid wires being secured to a frame which is annular in shape, but the invention may be applied to grids which are other than circular, such as rectangular, in plan.

In Figures 1 and 2 the grid wires 1 are secured to an annular frame by being brazed, welded, or otherwise suitably secured between an annular ring 2 and an internally directed flange 3 on a supporting member 4 having a cylindrical portion 5. The supporting member may take other forms, and in fact it is not an essential part of the invention. Grids can be made and prestressed as described below using only the annular ring 2 to support the wires. The wires are tensioned by any one of the well-known methods, as, for example, by expanding the support 4 and ring 2, the tension being sufiicient to enable the grid to withstand the rated dissipation but being within the elastic limit of the wires. In accordance with the invention the frame formed by the annular member 2 and flange 3 is then deformed or dished upwardly, that is, away from the direction of the cathode or other electrode to which it will be closely adjacent when assembled in the electron tube, as shown in Figure 3 so that, instead of the frame lying in approximately the same plane as the grid wires 1, it lies along the surface of a shallow cone, the wires 1 between the frame remaining straight and parallel to the base of the cone. By this means a considerable stress is applied to the wires immediately adjacent to the frame. The wires remain substantially straight over their unsupported length between the frame, but the stress applied ensures that if a Wire is strongly heated, particularly over the central portion, the deformation resulting from the consequent expansion of the wire is toward the apex of the cone to which frame 2, 3 has been formed. It is not essential that the wires 1 be tensioned before the frame.

is dished; alternatively it may be convenient to dish the frame after the wires have been secured to it with but moderate tension, and thereafter fully tension the wires.

An alternative grid construction in accordance with this invention is shown in Figure 4. In this embodiment the wires forming the grid 1 are brazed or welded between the annular member 2 and the flange 3 as described with reference to Figures 1 and 2, but with only moderate wire tension. The wires are then each formed as shown in Figure 4 so that there is a sharp bend 6 adjacent to the supporting members 2 and 3, and then the required wire tension is applied as by stretching the frame formed by the members 2 and 3. In this construction also the wires are substantially straight over most of their length, but the stress in the wire adjacent to the supporting members ensures that, should the tension be reduced or the wire break, the distortion will be in the direction of the original preform, that is, away from the cathode.

What is claimed is:

1. A planar type of grid comprising a frame which is an annulus, said frame having a first and second side, a plurality of substantially parallel co-planar spaced resilient wires extending across said frame and having a Patented .Nov. 18, 1958- said frame, said lengths being ofiest at an angle to the common plane of said wires in a direction away from said second side of said f ame, said wires being main,- tained substantially straight whileunder tension'whereby said wires will deflect in a direction away from'said second' side of said frame upon decrease of said tension.

2. A planar type of grid comprising a frame which is an annulus, said frame having a fiirst and second side, a plurality of substantially parallel spaced, resilient Wires lying in a first plane and having their end portions attached to a first'side of 'said frame, said end portions and said frame lying in a. second plane parallel to said first plane, said wires being stressed in a direction away from said second plane wherebyany decrease in tension of said wires will result in a deflection thereof in a direction away from said second side of said frame.

3. A planar type' of grid as claimed in claim 2 wherein the frame consists of two members, the grid wires being secured there-between.

4. A planar type of grid comprising a frame having first and second sides, a plurality of substantially par 'allel co-planar spaced resilient wires extending across said frame and having their end portions secured to said first side of said frame, said end portions being offset at an angle to the common plane of said wires in a direction opposite to said second side of said frame whereby each of said wires remains substantially straight while under tension and will deflect in a direction away from the said second side of said frame upon decrease of said tension.

5. A method of making a. planar type of grid for use in an electron tube, said tube having an electrode "anda grid closely spaced to the electrode comprising the steps of securing substantially straight lengths of grid wires to a frame so that they lie substantially in a common plane, stressing the wires adjacent to the frame in a common direction, and tensioniiig the wires so that, the wires between the frames will remain substantially straight in its cold state whereby saidwires' upon expansion due to heating will deflect in saidv common. direction.-

6. A method as claimed in claim 5 wherein the grid wires are-stressed by dishing the frame upwardly along its inner circumference in a direction away from said electrode.

7. A method as claimed in claim 5 wherein the grid wires are secured to an annular frame, and thereafter the frame is dished to fo'rmpart of the surface of a shallow cone with the grid wires between the frame being parallel to the base of the cone.

8. A method as; claimed in claim 7 wherein the grid wires are tensioned before the frame is dished.

9. A method; aszclaimed, in claim 5 wherein the grid wires are s ressed. by deforming the grid wires to form sharp bends therein adjacent the frame, the deformation being away' from the. direction of the. said closely spaced other electrode, and are thereafter tensioned by stretch ing the. frame;

References- Cited in the file of this patent UNITED STATES PATENTS 2,282,392'- Bieling May 12, 1942 2,397,233v Bingley Mar. 26,v 1946 2,455,863 Koch Dec. 7, 1948 2,495,259 Jackson ..a Jan. 24, 1950 2,610,387 Borland et al Sept. 16, 1952 2,654,940, Law e Oct. 13, 1953 

